DC converters can be used in many different applications to provide controlled or regulated voltages.
One such application, in the context of which the present invention is described by way of example below but to which the invention is not in any way limited, is for producing three relatively high voltages, referred to as voltages VSET, VERASE, and VSCAN, which are used respectively to set, erase, and scan a plasma display panel (PDP) in equipment such as a television set. For example the voltage VSET may be between 180 and 215 volts, the voltage VERASE may be between 70 and 120 volts, and the voltage VSCAN may be between −160 and −230 volts. Power levels associated with these voltages depend on the type of PDP but may typically be in a range of 6 watts to about 30 watts for each voltage, with a combined maximum power of about 30 to about 60 watts.
A known PDP power supply typically generates other secondary voltages which are isolated from a rectified a.c. supply used for powering the equipment. The relatively high voltages can be generated from one or more of these other secondary voltages; consequently isolation between the inputs and outputs of DC converters producing these relatively high voltages may not be required.
Each of these relatively high voltages is required to be independently adjustable within a relatively wide range, with the adjustments being in accordance with requirements and characteristics of the individual PDP assembled in an individual item of equipment. Thus for television sets, the adjustment of each of these voltages is carried out during assembly of each individual television set.
The requirement to adjust each of these voltages independently prevents the use, for producing and regulating these voltages, of a single multiple-output DC converter in which one output voltage is regulated by controlling a primary switch of the converter and the other output voltages are regulated by means of cross coupling, using either coupled windings in a flyback converter or coupled inductors in a forward converter.
The power levels associated with these voltages are too high for it to be practical to regulate one voltage using a primary switch of a DC converter and to use linear regulators for post regulation for the other voltages. In addition, the relatively high voltages make the use of buck type regulators complicated and not cost effective, and boost type converters are not practical due to the lack of a current limit.
A known approach to providing and regulating these three relatively high voltages is to use three completely independent flyback converters. For high volume production and for a low cost driven market, such as is the case for television sets, this approach is not very cost effective.
It is known to provide a flyback converter with a plurality of outputs which can be derived from one or more secondary windings of a transformer of the converter, and it is known to seek to regulate such multiple outputs independently.
For example, Bourdillon U.S. Pat. No. 6,552,917 issued Apr. 23, 2003 and entitled “System And Method For Regulating Multiple Outputs In A DC-DC Converter” discloses a multiple output flyback converter in which in each switching cycle a primary switch is first closed to store energy in a transformer, and then a plurality of secondary switches associated with respective outputs are closed in sequence to supply energy from the transformer to respective outputs. Fast feedback loop controllers in the output circuits control the switching of the secondary switches, and a slow feedback loop controller controls the switching of the primary switch.
In this known converter the fast secondary feedback loops only compensate for small load changes of the order of 1 to 5 percent, and the primary switch feedback loop operates slowly. Such a converter does not provide independent regulation of the outputs for large and rapid load changes, because the energy supplied to the transformer in each switching cycle is shared among the plurality of outputs.
Accordingly, such a converter is not suited to producing and regulating the relatively high voltages of a PDP, for which it has been found that the loads can change rapidly by large amounts. For example, currents associated with these voltages may include short current pulses, for example with durations of the order of 20 μs or less and peak amplitudes of the order of ten times an average output current.
Bourdillon U.S. Pat. No. 6,606,257 issued Aug. 12, 2003 and entitled “Independent Regulation Of Multiple Outputs In A Soft-Switching Multiple-Output Flyback Converter” describes another flyback converter with multiple outputs derived from one or more transformer secondary windings. Again in this case in each switching cycle first a primary switch is closed to store energy in a transformer, and then a plurality of secondary switches associated with respective outputs are closed in sequence to supply energy from the transformer to respective outputs. Thus again in this case the energy stored in the transformer in the first part of a switching cycle is shared among the outputs in the remainder of the switching cycle, so that independent regulation of the output voltages is ineffective for rapid and large changes of the output loads.
Various other DC converter or regulator arrangements, having two or more outputs with some degree of regulation, which have similar or other shortcomings or limitations, are known for example from the following references:
Jiang U.S. Pat. No. 5,862,042 issued Jan. 19, 1999 and entitled “Multiple Output DC To DC Converter”, in which multiple output voltages are produced by respective output circuits which are coupled to respective input circuits all of which share a common input subcircuit for example including a switch operating at a fixed duty cycle. Each output is regulated using its own switch controlled by its own PWM (pulse width modulation) controller; the converter thus requires as many PWM controllers and associated feedback loops as there are output voltages.
Rozman U.S. Pat. No. 6,058,026 issued May 2, 2000 and entitled “Multiple Output Converter Having A Single Transformer Winding And Independent Output Regulation”, in which two output voltages are provided and are regulated by two PWM controllers, one controlling a primary switching circuit and the other controlling a switch on the secondary side of a transformer.
Blair U.S. Pat. No. 6,211,579 issued Apr. 3, 2001 and entitled “Multiple Output Converter Having A Low Power Dissipation Cross Regulation Compensation Circuit”, in which an adjustable impedance is provided in an output voltage path for which regulation is provided by a feedback loop, and is controlled in dependence upon sensed output voltages of other output voltage paths.
Ivanov U.S. Pat. No. 6,522,110, issued Feb. 18, 2003 and entitled “Multiple Output Switching Regulator”, in which switches of a voltage regulator are controlled by a decision logic block to control the flow of current from an inductor, to which energy is supplied via a power switch, to any of two or more outputs whose voltages are monitored by error amplifiers which supply error signals to the decision logic block.
“Low Power Boost Converter For Portable Applications”, by Eddy Wells and Mark Jordan, Texas Instruments Incorporated, 2001, which describes a UCC3941 synchronous boost converter as incorporating a PFM (pulsed frequency modulation) control technique and a multiplexed coil technique to generate multiple outputs from a single inductor, energy pulses stored in the inductor being time shared among the outputs in accordance with a predetermined priority scheme.
ON Semiconductor Data Sheet NCP4326, “Secondary Controller For Multi-Output Quasi-Resonant Switchmode Power Supplies”, September 2005, which discloses a converter with one output voltage regulated by a primary PWM controller which controls a primary switch and one or two other output voltages regulated using secondary switches controlled by a secondary controller.
Despite these numerous approaches, a problem of independent regulation of multiple outputs of a DC converter remains, especially where there may be rapid and large changes of load currents supplied via the respective outputs. This problem can apply in various applications of converters, and is not limited to any particular application of a converter, such as for producing high voltages for a PDP as discussed above.
There is therefore a need to provide an improved DC converter with independently regulated multiple outputs.